Hybrid bearings have been widely used in many applications ranging from machine tool spindles to high-speed turbomachinery. Particularly in machine tool spindles that use air or water as the lubricating fluid, the bearings are likely to experience deleterious combined hydrostatic and hydrodynamic effects. The speed with which these spindles can operate is often considerably less than the desired speed because of the stability limitations of the bearings. A bearing with enhanced stability, such as provided by the present invention, later explained, would enable higher speeds and advance the start-of-the-art in high-speed machining applications. In optics grinding applications, for example, the ability to achieve higher speeds would enable a higher level surface finish and a greater quality of optics. In milling applications, as another example, the ability to achieve higher spindle speeds would enable higher rates of material removal and lower manufacturing costs.
There are many turbomachinery applications, furthermore, in which hybrid bearings are attractive. Hybrid fluid film bearings are predominantly used in turbopumps used to pump cryogenic fluids such as liquid oxygen and liquid hydrogen (See, for example, "Hydrostatic Bearings for Cryogenic Rocket Engines" by J. M. Reddecliff and J. H. Vohr, Journal of Lubrication Technology, July 1969, pp. 557-575). Again, however, such machines are limited in their speed capabilities due to stability limitations of the bearings. An improved bearing that could be used to achieve higher speeds would also enable such machines to be built with reduced size and weight and higher pump efficiencies.
Conventional hybrid bearings utilize orifices or capillaries to compensate multi-recess journal bearings with, generally, rectangular recesses. The recesses help to establish more uniform hydrostatic pressure as well as interrupt the circumferential fluid flow to reduce the destabilizing hydrodynamic bearing forces generated by the rotation of the shaft. Destabilizing forces, however, are prevalent in these bearings and so they are limited in the maximum speed that they can achieve. Researchers and inventors have focused on improving the stability of these bearings in recent years; and although some improvement has been made in conventional design by orienting the orifices at an angle and by making minor changes to the shape of the bearing recesses (See, for example, "Experimental Test Results for Four High-Speed, High-Pressure, Orifice-Compensated Hybrid Bearings" by N. M. Franchek and D. W. Childs, Journal of Tribology, Vol. 116, January 1994, pp. 147-153), stability problems still exist.